The United States and its NATO allies are close to an agreement to erect a missile defence shield over Europe, a project that would give the military alliance a fresh purpose while testing President Barack Obama's campaign to improve relations with Russia.

The deal is likely to be sealed at a two-day NATO summit starting Friday in Lisbon, officials say, as part of what the alliance calls its new strategic concept — the first overhaul of its basic mission since 1999.

The summit will include Obama, Prime Minister Stephen Harper and leaders of the 26 other member countries of the North Atlantic Treaty Organization. Russian President Dmitry Medvedev will attend a separate NATO-Russia session on Saturday.

Outlines of the missile-defence arrangement were provided to The Associated Press by American officials who spoke on condition of anonymity.

Under the plan, a limited system of American missile interceptors and radar installations already planned for southeastern Europe would be linked to expanded European-owned missile defences. That would create a broad system to protect every NATO country against medium-range missile attack.

Russia to be invited in

NATO plans to invite Russia to join the missile shield effort, although Moscow would not be given joint control. The gesture would mark a historic milestone for the alliance, created after the Second World War to defend Western Europe against the threat of an invasion by Soviet forces.

The previous American administration of George W. Bush proposed stationing 10 interceptor missiles in Poland and an advanced radar in the Czech Republic, saying the system was aimed at blunting future missile threats from Iran. Russia was furious, saying the missiles threatened the deterrent value of its nuclear arsenal. At one point Moscow warned that if the plan went forward it would station missiles close to Poland.

The Obama administration cancelled the original plan in September 2009, and later proposed building elements of the missile shield in Bulgaria and Romania, two NATO countries closer to Iran. The U.S. has asked Turkey, a member of NATO, to host some of the radar defences, but Turkey has hesitated, saying it does not want the system explicitly to target its neighbour, Iran.

U.S. officials close to pre-summit talks were optimistic that the proposed European missile shield's remaining obstacles could be overcome. They said Russia seems to be seriously considering NATO's plan, while Turkey's concerns could be finessed.

"The Russians seem to be playing ball and seem to be somewhat open-minded about this," said Stephen Larrabee, a specialist in European security issues at the Rand Corporation think-tank. In Larrabee's view, though, NATO must still persuade Moscow that the planned system will not undermine the credibility of Russia's nuclear arsenal. "I don't think they've been completely convinced of that yet," he said.

Other experts agreed that Turkey's concerns about singling out Iran could probably be answered. "I would be surprised if this proved to be a deal-breaker," said Steven Pifer, a Russian affairs specialist at the Brookings Institution.

Afghan security on agenda

NATO leaders also are expected to adopt a shift in responsibility for Afghanistan's security from U.S.-led NATO forces to the Afghans, beginning in the first half of next year and finishing in 2014. Afghan President Hamid Karzai is expected to attend one NATO session.

Saturday's NATO-Russia meeting is expected to discuss a bigger Russian role in the Afghan conflict. A NATO spokesman has said Russia has been asked to contribute about 20 transport helicopters and provide training for Afghan helicopter pilots.

In adopting the new strategic concept, NATO is trying to adapt to 21st-century security threats. In Europe, "there is less fear of foreign intervention or aggression than there ever has been before in the history of the North Atlantic alliance," Wolfgang Ischinger, a former German ambassador to the U.S., told a recent conference.

Europe's sense of security improves chances for better relations with Moscow. But it also has encouraged European governments to slash military spending, and constructing a Europe-wide missile defence system would be costly.

The allies are expected to declare in Lisbon that nuclear weapons will remain a central element in NATO defences. At the same time, the alliance will urge Russia to enter into negotiations to reduce short-range nuclear weapons in Europe. Prospects for those talks appear dim, however, unless the U.S. Senate ratifies the new Strategic Arms Reduction Treaty, which would cut the number of American and Russian long-range nuclear weapons. Republican gains in the recent congressional elections have made ratification more doubtful.